Acoustic diaphragms of soft material, such as paper or composites of a soft material and phenolic, have been most commonly adopted. The softness tends to damp out local area oscillations along the material which tend to have a negative effect on the frequency response of the diaphragm. Also, because of its softness, the material itself does not generate a sound as a result of the movement of its structure in connection with such local area oscillations.
The strength and stiffness requirements of diaphragms, and the relatively lesser degree of strength of paper or other soft materials, typically leads to relatively larger masses for diaphragms made of such materials. Since there generally is a mass-related frequency response roll-off at the upper frequency end for a diaphragm, this mass factor typically results in a roll-off in the frequency response at a significantly lower frequency than would otherwise be achieved.
For example, paper diaphragms which are used for frequencies from about 5 kilohertz to about 20 kilohertz (the upper part of the audio range for humans) typically have responses which roll-off significantly below the 20 kilohertz point.
Much effort has gone into attempting to improve various forms of paper or other soft diaphragms. Thus, relatively strong paper diaphragms of less than typical mass have been made. By way of example, in cone-shaped diaphragms of relatively large size, thus particularly adapted for the lower end of the audio spectrum, ridges have been provided to increase effectiveness. The usefulness and role of such in cone-shaped paper diaphragms having base diameters as small as about 12 inches (30.5 centimeters) has been significantly recognized.
Hard diaphragms of metallic material, for example having a dome shape, have been used to a generally lesser extent than soft diaphragms. Metal can typically provide more strength for the same mass than paper or other soft material. Thus, metallic material is advantageous with regard to roll-off at the high frequency end of a diaphragm's response.
However, metal diaphragms are recognized as presenting practical problems in formation for manufacturing purposes. For example, the thin metal tends to break during formation in a cold die, and such tendency can only be enhanced by complexity in the form of the structure. Formation in a hot die overcomes this, but does incorporate additional expense in constructing the hot die and also brings some negative safety considerations into the manufacturing process.
In addition, metallic diaphragms do not incorporate the damping out, by the material, of local area oscillations, as occurs for paper or other soft materials. The structural oscillations of the metal as a result of such local area oscillations, most particularly where the oscillations are resonances, also have a negative impact on the sound generated by metallic diaphragms. Specifically, "chirps" stemming from these relatively low level localized resonances, result from the hard, unyielding nature of the material, and interfere with the performance of such diaphragms, particularly in respect to people with acute hearing.
The present invention combines significant advantages typically associated with hard as well as soft material diaphragms. In so combining such advantages, it is most directly of concern with reference to diaphragms of relatively small and intermediate size.